It makes perfect sense to me. (Feels incredibly relatable, in fact) Mitsuki is a pretty typical artist, being jerked around by feelings that feel bigger than she is. She started feeling like a fraud, because she was playing music that she didn't "feel" in order to fit into her band, because she was afraid to try and make it on her own. She already felt like a fraud. When Aya saw her playing, and she saw Aya's expression (of concern, but to Mitsuki it must've felt like disappointment,) it made her feel like it was confirmed, the jig was up, and it was just a matter of time before Aya dumped her for somebody "real". When somebody is depressed, actually depressed, they have the least charity for themselves. Depression lies, and that's where Mitsuki's been living, under the lies her depression was badgering her with. Aya adores her, though, honestly, and has been trying to wait for her to open up, move on, anything really, but she didn't want to push.
Mitsuki let her insecurities come out to Aki, though, and it hit Aki at a really bad time, so when Aki's friend snubbed him, he started spiraling too. When the kid went missing, maybe because of what Mitsuki said to him, well, a moment like that has a way of snapping a lot of things into focus. Mitsuki is actually really good in a crisis, and so that got her confidence back, so she played, because when that fight or flight response actually kicked in, she's in it to win it. When she played, from the heart, and saw how much Aya loved the way she actually played, she got overwhelmed with feels in the moment and proposed on impulse.
Because honestly, who could look at somebody who is looking at you like that and not want to spend the rest of your life making them look like that?
Very definitely. A large proportion of the people in my life have been artists of various kinds at various levels. Big emotions often lead to blurting. And of course our mangaka is an artist, who would have such associations and such insights . . .
Of course our central pair need to work on things. With humans problems never just go away, they just become more manageable with time/wisdom (if we’re lucky).
I hope that Oosawa-san realises that there’s an alcohol problem though; it’s been blunting her natural courage and resourcefulness. Koga-san is going to need her partner’s strength, because that one is definitely not out of the woods yet (and may always remain something of a woodland creature . . . )
. And of course I’m invested in the other characters with their own longings and such. Although I could imagine Arai-sensei might eventually want to move on to other projects, or indeed take a sabbatical, at some point.